I have timer to emit signal to trigger sendMessage every 30 millisecond. after hours , i got heap corruptions

it said
Windows has triggered a breakpoint in matx_migrate.exe.

This may be due to a corruption of the heap, and indicates a bug in
matx_migrate.exe or any of the DLLs it has loaded.

The output window may have more diagnostic information

Qt Code: 
  1. void ClientThread::sendMessage(){
  2.   #ifdef _DEBUG
  3.   qWarning("sendMessage %d\n", QThread::currentThreadId()); 
  4. #endif
  5.   numOfMsg++;
  6.   if(numOfMsg>CHECK_PERIOD){
  7.     sendCheckMessage();
  8.     numOfMsg=0;
  9.   }
  10.   QList<QByteArray> dataToSend= cannonPtr->getDataToSend();
  11.   if(!dataToSend.isEmpty()){
  12.     QByteArray data;
  13.     QDataStream out( &data, QIODevice::ReadWrite );
  14.     out.setVersion(QDataStream::Qt_4_4);
  15.     out << (quint16)0;//reserve for size
  16.     out << (quint16)dataToSend.count();
  17.     for(int i=0; i<dataToSend.count(); i++){
  18.  
  19.       data.append(dataToSend[i]);
  20.       //out<<dataToSend[i];
  21.  
  22.     }
  23.     out.device()->seek(0);
  24.     out<< (quint16)(data.size() - sizeof(quint16));
  25.     #ifdef _DEBUG
  26.     quint16 size;
  27.     quint16 num;
  28.     quint16 kind;
  29.     qint32 frame;
  30.     qint32 id;
  31.     qint32 x;
  32.     qint32 y;
  33.     bool dead;
  34.     QTime time;
  35.     out.device()->seek(0);
  36.     out>>size>>num>>kind>>frame>>id>>x>>y>>dead>>time;
  37.     qWarning()<<"size: "<<size<<"num: "<<num<<"frame: "<<frame; 
  38. #endif
  39.     socketPrt->write(data);
  40.     socketPrt->waitForBytesWritten();
  41.  
  42.     dataToSend.clear();
  43.   }
  44. }
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any comments thanks very much